1. Field of the Invention
The present invention relates to methods for preparing agglomerated pellets of polytetrafluoroethylene, including agglomerated pellets of polytetrafluorethylene which contains one or more fillers, and the agglomerated pellets of polytetrafluoroethylene so produced. The present invention also relates to methods for preparing molded articles of polytetrafluoroethylene, including molded articles of polytetrafluoroethylene which contains one or more fillers, and the molded articles prepared thereby.
2. Discussion of the Background
Polytetrafluoroethylene (herein after referred to as “PTFE”) is a useful material for the formation of molded articles. In particular, PTFE which may contain a filler such as bronze or graphite, has found application in the automotive and mechanical fields for the manufacture of piston rings and seals.
However, it is difficult to use finely powdered PTFE in applications which involve small or intricate molds. Specifically, the poor flow properties and low density of finely powdered PTFE adversely affect its ability to completely fill small or intricate molds and to afford suitable molded articles.
In response to the problems experienced with finely powdered PTFE, pelletized PTFE has been developed. PTFE pellets, also referred to as agglomerated PTFE, exhibits much better flow properties and a higher bulk density, as compared to finely powdered PTFE, and can thus be successfully used with small and intricate molds, such as molds for thin-walled sleeves.
Conventionally, PTFE has been pelletized or agglomerated by means of a process in which PTFE powder is treated with a solvent system, sometimes referred to as a wetting agent, to effect agglomeration and then dried to afford dried pellets. For example, U.S. Pat. No. 5,182,119 discloses an apparatus and method for the production of agglomerated pellets from PTFE resin powder, and discloses the use of trichloroethylene, alkyl alcohols, and water soluble alcohols plus water as the wetting agent.
U.S. Pat. No. 6,203,733 discloses a process for producing granulated PTFE powder by coagulating PTFE powder with a liquid medium containing water and a perfluoroalkyl alkyl ether. The PTFE primary powder is mixed with the liquid medium and stirred at a certain intensity to obtain the desired degree of coagulation and granulation.
U.S. Pat. Nos.4,370,436 and 5,321,059 disclose a process for preparing a filled agglomerated PTFE molding powder by agitating finely-divided PTFE particles and a surface-treated filler in an aqueous medium, which includes a water-immiscible organic liquid.
U.S. Pat. No. 6,197,862 discloses a process for preparing a filled PTFE granular powder by stirring PTFE powder, a filler, and water in the presence of an organic liquid and a non-ionic solvent.
U.S. Pat. No. 6,281,281 discloses a process for preparing a granulated powder of filled PTFE by wetting 100 parts by weight of a mixture of PTFE powder and filler with 30 to 60 parts by weight of an aqueous solution containing an anionic surfactant in a specified concentration.
U.S. Pat. No. 6,350,801 discloses the preparation of a filled granular PTFE powder by forming an aqueous slurry of PTFE powder and a filler by using an organic liquid which forms a liquid-liquid interface with water and a nonionic surfactant and the granulating in combination of stirring and de-agglomeration.
U.S. Pat. No. 3,597,405 describes a method of agglomeration in which the polymer powder is wetted with a minor amount of a liquid having a low surface tension and the wetted powder is formed into agglomerates by, for example, a tumbling operation. After formation of the agglomerates, the liquid is removed by heating.
U.S. Pat. No. 3,682,859 discloses a similar process in which the tetrafluororoethylene polymer is wetted with an organic liquid. U.S. Pat. No. 3,882,217 describes an agglomeration process in which the tetrafluoroethylene polymer product is wetted with a small amount of a solution of a volatile non-ionic surfactant in water prior to agglomeration. U.S. Pat. No. 3,983,200 discloses a process in which tetrafluoroethylene polymer is first wetted with an aqueous solution containing a protective colloid.
In the past, particularly good results have been achieved by agglomerating PTFE powders and a filler in the presence of a mixture of water and ethanol. However, recent environmental concerns and the desire to reduce emissions of volatile solvents, such as ethanol, have prompted the search for suitable non-volatile solvent replacements for ethanol. In this regard, mixtures of water and dipropylene glycol n-butyl ether (“DPnB”) have been used as the solvent system or wetting agent for the agglomeration of PTFE. However, it has been found that the PTFE pellets prepared with aqueous DPnB are inferior as compared to those prepared with aqueous ethanol. In particular, the molded articles prepared from pellets of filled PTFE prepared with a DPnB-water mixture exhibit a high degree of inhomogeneity as compared to those prepared with aqueous ethanol. In addition, molded articles prepared from such PTFE pellets can exhibit spotting.
The bulk density of the PTFE pellets is also important when preparing molding articles such as thin-walled sleeves. Such sleeves are typically manufactured by cutting a longer sleeve to the desired length on a lathe. When working with a sleeve on a lathe, a fixed length of waste material is inevitably generated at each end of the longer sleeve when it is mounted on the lathe via the chucks. PTFE particles with a low bulk density give a shorter starting sleeve and, thus, a high proportion of waste when the starting piece is cut to size on the lathe. In contrast, PTFE particles with a high bulk density afford longer starting pieces and a smaller percentage of waste upon subsequent cutting on a lathe.
The flowability of the PTFE particles is another important consideration. PTFE particles with low flowability pose problems when manufacturing molded articles, especially when using automated molded apparatus, such as failure to completely fill the mold. Incomplete filling of the mold can result in shorter products and the concomitant problems described above. PTFE pellets with good flowability are characterized by a narrow, uniform size distribution, approximate spherical shape, high hardness, and a lack of stickiness.
The absence of spotting in the PTFE pellets and the resulting molded articles is another important consideration. PTFE pellets and the molded articles which exhibit spotting are less desirable at least for aesthetic reasons. In addition, such spotting reveals a degree of inhomogeneity which may result in inferior physical and mechanical properties of the molded article, such as tensile strength, etc.
Thus, there remains a need for a method of preparing agglomerated pellets of PTFE which affords pellets which exhibit improved bulk density. There also remains a need for a method for preparing agglomerated pellets of PTFE which exhibit improved flowability. There also remains a need for a method for preparing agglomerated pellets of PTFE which exhibit a reduced spotting. There also remains a need for the pellets prepared by such a process. There also remains a need for a method for preparing molded articles from such agglomerated pellets of PTFE and the molded articles so produced.